As is known in the art, air traffic control (ATC) systems promote the safe, orderly and expeditious flow of aircraft traffic. Safety is principally a matter of preventing collisions with other aircraft, obstructions, and the ground, assisting aircraft in avoiding hazardous weather, assuring that aircraft do not operate in airspace where operations are prohibited, and assisting aircraft in distress.
As is also known, ATC systems employ information from both ground-based radar and aircraft-based transponders to indicate the horizontal and vertical position of one or more aircraft. Aircraft can include a so-called mode C transponder. The aircraft based Mode C transponder, upon interrogation by an ATC system, transmits information to the ATC about the altitude of the aircraft. The altitude determining method of existing ATC systems uses both Mode C data from the aircraft transponder as well as information from a multi-radar tracker (MRT) process. Data from the Mode C transponder and from the MRT are processed by an altitude post processor (APP) that further enhances the vertical mode of flight (MOF) determination, the determination being that of level or non-level flight.
Existing Mode C data has a resolution of 100 feet, due in part to low resolution of the received Mode C transponder data combined with further limitation of the existing APP processing method. The 100 foot resolution resembles a step function with 100 foot reported altitude jumps as the altitude crosses resolution boundaries. This resolution limit and finite altitude jumps result in both a delay time in the determination of the start or stop of an aircraft altitude change and a relatively high level of uncertainty as to the instantaneous aircraft rate of ascent or decent.
Due to the 100 foot altitude uncertainty, it is necessary to set a minimum limit of 200 feet before an altitude change can be reported by the conventional APPs. It is also necessary to delay altitude change determinations so as to filter out the instantaneous 100 foot uncertainty. Data with this uncertainty is presented to existing Conflict Alert (CA) and Minimum Safe Altitude Warning (MSAW) systems, which are utilized in existing ATC systems. The altitude determination time delay and the altitude uncertainty cause the CA and the (MSAW) systems to miss some real aircraft position conflicts and also to falsely declare some such conflicts. The safety of aircraft monitored by such ATC systems is thereby compromised.
Unlike horizontal motion, which is always non-stationary, vertical motion can be either stationary or non-stationary. Unfortunately, predictor-corrector algorithms, such as Kalman Filters, are incapable of producing precise zero-velocity estimates for stationary targets, for a number of reasons. One is that these algorithms use exponentially-fading historical data, which are slow to respond to fast changes in modes of flight. Another reason is that Mode C data are quantized, exhibiting frequent 100-foot jumps during level flight, instead of constant altitude. The quantization also produces staircase-like behavior during vertical motion, especially at speeds below 500 ft/min.
Because rapid response to changes in mode of flight is critical to the operation of the safety functions, two known methods have been employed to augment the performance of the ATC system tracker. One method retains a received altitude as a reference and then compares it to subsequent measurements. The method assumes level flight as long as subsequent altitudes do not deviate from the reference altitude by more than 200 feet. The reference altitude is reset to the most recent one after 65 seconds or when the 200-ft threshold is exceeded. If the reset occurs simultaneously with a 100-foot quantization jump, it can lead to a 300-ft. delay before vertical motion is declared. Another method employs a tracker-aided statistical analysis to calculate a probability that an aircraft is moving vertically. Non-level flight is declared when the probability exceeds 0.5. The response time of this method is shorter than the former by up to 40 percent.
Although the first method is quite effective at high speeds, its response time diminishes at low speeds because it takes longer to move 200 feet. At 200 ft/min, a 200-ft. move takes 60 seconds. If the data is provided by short-range radar (SRR) with a nominal scan period of 5 seconds, the detection delay would be 12 scans. The second method, though faster, is still not fast enough, according to ARCON Corp., for example, which sent to the Federal Aviation Administration (FAA) two memoranda about this issue. In one memorandum [MO18A-05, Nov. 4, 2005], it compared the warning time obtained after a vertical-motion is reversed from descent to ascent, to that obtained after a transition from level flight to descent, and found that the former was longer. The second memorandum [MO17A-05, Nov. 8, 2005] analyzed the delay encountered in detecting a transition from level flight to a 300-ft/min descent and claimed that the detection delay was longer than it should have been.